The present invention relates to the discovery that trialkyltin hydrides and dialkyltin dihydrides display selectivity with respect to both (a) the preferential reduction, under mild conditions, of one halogen to the exclusion of another halogen in a mixed halodisilane with preservation of the silicon-silicon bond and (b) the sequential reduction of halogen atoms in halodisilanes having two or more atoms of the same halogen. The products formed by reducing the halodisilanes in this manner are suitable as precursors for preparing amorphous silicon films for use as semiconductors and protective coatings.
Substitution of hydrogen for chlorine in chlorosilanes such as (CH.sub.3)SiCl.sub.3 is known and can be effected via reaction with trialkylsilanes in the presence of catalytic quantities of aluminum chloride. However, this catalyst is notorious in promoting skeletal rearrangements (i.e., redistributions involving Si-C and/or Si-Si bonds). The latter behavior makes this reaction unsuitable for reduction of halodisilanes when one wants to avoid skeletal rearrangements.
Organotin hydrides are known as effective reducing agents in organic chemistry and their reactivity patterns have been well documented. A common use of organotin hydrides is in the reduction of organic halides. EQU (4-n)RX+R.sub.n SnH.sub.4-n .fwdarw.(4-n)RH+R.sub.n SnX.sub.4-n
The above reaction has wide scope since R can be alkyl, aryl, acyl, propargyl, etc. The reaction conditions and efficiencies will vary with the R group (e.g., aryl halides are more difficult to reduce than alkyl halides) and with the halide (for alkyl halides the order of reactivity is RI&gt;RBr&gt;RCl&gt;&gt;RF). However, these reactions are known to proceed through free radical mechanisms and require the addition of free radical initiators.
U.S. Pat. No. 4,617,357, issued Oct. 14, 1986 to Pallie, et al., teaches the reduction of residual chlorine in glycidyl compounds with certain organotin hydrides without the glycidyl group thereby being attacked.
Grady, et al., J. ORG. CHEM., v. 34, p. 2014-2016, 1969, teaches the use of organotin hydrides for the sequential free radical reduction of organic halides, such as geminal dihalides, and simple carbonyl compounds. Grady, et al., does not teach the sequential or other reduction of halodisilanes, nor does it teach reduction methods in the absence of free radicals.
Organotin hydrides typically react with organic halides and unsaturated bonds via a free radical mechanism. These reactions either require heating or initiation by a free radical source. Free radicals are well known to attack silicon-silicon bonds and are therefore not appropriate reactive intermediates in reductions of disilanes when skeletal arrangements of silicon-silicon bonds are to be avoided.
Hiiro, et al., U.S. Pat. No. 4,115,426, issued Sept. 19, 1978, teaches a method of preparing dialkylchlorosilanes by the sequential reduction of the corresponding dialkyldichlorosilane. The reducing agents in the preparation are sodium borohydride and sodium hydride. Hiiro, et al., does not teach the reduction of silanes which do not contain alkyl groups nor the reduction of halodisilanes.
Similarly, Wilt, et al., J. AM. CHEM. SOC., vol. 105, p. 5665-5675, 1983, teaches the reduction of silicon-containing organic halides by organotin hydrides but is limited to the reduction of alpha-halosilanes (i.e., the halogen atom is bonded to a carbon atom adjacent to a silicon atom but is not bonded directly to a silicon atom).
Aono et al., U.S. Pat. No. 4,639,361, issued Jan. 27, 1987, teaches a process of preparing disilane Si.sub.2 H.sub.6 from the reduction of hexachlorodisilane Si.sub.2 Cl.sub.6 with a mixture of LiH and LiAlH.sub.4.
U.S. Pat. No. 3,439,008, issued Apr. 15, 1969 to Berger, teaches a method of reacting an organostannane and a halogenated Group IVb elemental material selected from the group consisting of a halosilane, a halogermane, and a halosiloxane. However, Berger requires the presence of a Group III Lewis acid promoter such as Al.sub.2 (CH.sub.3).sub.6 and does not teach the selective, sequential reduction of halodisilanes.
Related U.S. patent application entitled Method of Forming Semiconducting Amorphous Silicon Films From the Thermal Decomposition of Fluorohalodisilanes, Ser. No. 064,641, filed June 22, 1987, by the same inventors as this application, describes a use for fluorodisilanes such as those produced by the instant invention. The fluorodisilanes are used as precursors to form semiconducting films of amorphous silicon by a chemical vapor deposition process.
Related U.S. patent application entitled, Method of Selective Reduction of Polyhalosilanes with Alkyltin Hydrides, Ser. No. 078,444, filed July 27, 1987, by the same inventors as this application, discloses a method of reducing halosilanes by reacting at room temperature or below with alkyltin hydrides without the use of free radical intermediates. Alkyltin hydrides selectively and sequentially reduce the Si-Cl, Si-Br, or Si-I bonds while leaving intact any Si-F bonds. When two or more different halogens are present on the halosilane, the halogen with the highest atomic weight is preferentially reduced. However, there is no teaching of the selective, sequential reduction of halodisilanes, in which the easily destroyed silicon-silicon bond is present.
Thus, there exists no known method to effect preferential reduction of one halogen species to the exclusion of another in halodisilanes, such as SiF.sub.3 SiHBr.sub.2, to enable preparation of fluorodisilanes which are suitable as precursors for preparing amorphous silicon films for use as semiconductors and protective coatings. Further, this method of preferential reduction should not include free radicals or acidic or basic catalysts which are known to attack Si-Si bonds.